I. Field of the Invention
This invention relates to an AlxGayInzN (wherein 0 less than yxe2x89xa61 and x+y+z=1) semiconductor wafer having superior surface quality at its Ga-side, and to a method of fabricating such E wafer.
II. Description of the Related Art
GaN and related GaN-like III-V nitride crystal films, represented by the general formula AlxGayInzN, wherein 0 less than yxe2x89xa61 and x+y+z=1, are useful materials in various applications, such as high temperature electronics, power electronics, and optoelectronics (e.g., light emitting diodes (LEDs) and blue light laser diodes (LDs)). Blue light emitting diodes (LED""s) and lasers are an enabling technology, allowing much higher storage density in magneto-optic memories and CDROM""s and the construction of full color light emitting displays. Blue light emitting diodes may replace today""s incandescent light bulbs in road and railway signals etc., where they promise very substantial cost and energy savings.
Currently, AlxGayInzN films are grown on non-native substrates such as sapphire or silicon carbide due to unavailability of high quality AlxGayInzN substrates. However, differences in thermal expansion and lattice constants between such foreign substrates and the AlxGayInzN crystals epitaxially grown thereon cause significant thermal stress and internal stress in the grown AlxGayInzN crystals. The thermal stress and internal stress cause micro-cracks, distortions, and other defects in the AlxGayInzN crystals, and make such AlxGayInzN crystals easy to break. Growing on lattice non-matched foreign substrates causes high density of lattice defects, leading to poor device performance.
In order to reduce the deleterious thermal stress and high defect density in the grown AlxGayInzN crystals, it is desirable to provide high quality freestanding AlxGayInzN wafers as film-growing substrates in place of the above-mentioned foreign substrates.
U.S. Pat. No. 5,679,152 entitled xe2x80x9cMethod for Making a Single Crystal Ga*N Articlexe2x80x9d and U.S. Pat. No. 5,679,153 entitled xe2x80x9cBulk Single Crystal Gallium Nitride and Method of Making Samexe2x80x9d disclose a hydride vapor phase epitaxy (HVPE) process for fabricating freestanding AlxGayInzN crystals, which may advantageously be used as crystal-growing substrates for homoepitaxial growth of AlxGayInzN crystals thereon.
Since quality of a subsequently grown AlxGayInzN crystal is directly correlated to the quality of the substrate surface and near surface region on which the AlxGayInzN crystal is grown, it is important to provide a highly smooth initial substrate surface without any surface and subsurface damage.
However, after mechanical polishing, the AlxGayInzN crystals typically have very poor surface quality, with substantial surface and subsurface damage and polishing scratches. Additional wafer finish processing therefore is necessary to further enhance the surface quality of the freestanding AlxGayInzN crystal, so that it is suitable for high-quality epitaxial growth and device fabrication thereon.
Crystalline AlxGayInzN generally exists in a chemically stable wurtzite structure. The most common crystallographic orientation of AlxGayInzN compounds has two polar surfaces perpendicular to its c-axis: one side is N-terminated, and the other one is Ga-terminated (Ga hereinafter in the context of the Ga-side of the crystal structure being understood as generally illustrative and representative of alternative Group III (AlxGayInz) crystalline compositions, e.g., of a corresponding GaxIny-side in GaxInyN crystals, of a corresponding AlxGayInz-side in AlxGayInzN crystals, and of a corresponding AlxGay-side in AlxGayN crystals).
Crystal polarity strongly influences the growth morphology and chemical stability of the crystal surface. It has been determined that the N-side of the AlxGayInzN crystal is chemically reactive with KOH or NaOH-based solutions, whereas the Ga-side of such crystal is very stable and not reactive with most conventional chemical etchants. The N-side can therefore be easily polished, using an aqueous solution of KOH or NaOH, to remove surface damage and scratches left by the mechanical polishing process and to obtain a highly smooth surface.
The Ga-side (AlxGayInz side) of the AlxGayInzN crystal, on the other hand, remains substantially the same after contacting the KOH or NaOH solution, with its surface damage and scratches unaltered by such solution. See Weyher et al., xe2x80x9cChemical Polishing of Bulk and Epitaxial GaNxe2x80x9d, J. CRYSTAL GROWTH, vol. 182, pp. 17-22, 1997; also see Porowski et al. International Patent Application Publication No. WO 98/45511 entitled xe2x80x9cMechano-Chemical Polishing of Crystals and Epitaxial Layers of GaN and Ga1xe2x88x92xxe2x88x92yAlxInyNxe2x80x9d.
However, it has been determined that the Ga-side of the AlxGayInzN crystal is a better film-growing surface than the N-side. See Miskys et al., xe2x80x9cMOCVD-Epitaxy on Free-Standing HVPE-GaN Substratesxe2x80x9d, PHYS. STAT. SOL. (A), vol. 176, pp. 443-46, 1999. It therefore is important to provide a wafer finish process that is particularly effective for preparing the Ga-side of the AlxGayInzN crystal to make it suitable for subsequent crystal growth thereupon.
Reactive ion etching (RIE) recently has been used to remove a layer of surface material from the Ga-side of an AlxGayInzN wafer to obtain smoother wafer surface. See Karouta et al., xe2x80x9cFinal Polishing of Ga-Polar GaN Substrates Using Reactive Ion Etchingxe2x80x9d, J. ELECTRONIC MATERIALS, vol. 28, pp. 1448-51, 1999. However, such RIE process is unsatisfactory because it is ineffective for removing deeper scratches, and it introduces additional damage by ion bombardment and additional surface irregularities by concomitant contamination, which in turn requires additional cleaning of the GaN wafer in an O2 plasma.
It is therefore advantageous to provide an AlxGayInzN wafer with high surface quality on its Ga-side, with substantially no or little surface and subsurface damage or contamination. It is also desirable that such AlxGayInzN wafer is prepared by a surface polishing process that is both economic and effective, and requires no cumbersome cleaning process during or after polishing.
The present invention generally relates to an AlxGayInzN (wherein 0 less than yxe2x89xa61 and x+y+z=1) wafer having superior surface quality at its Ga-side, and to a method of fabricating such wafer.
One aspect of the present invention relates to a high quality AlxGayInzN wafer of such type, wherein the wafer has a surface roughness characterized by a root means square (RMS) roughness of less than 1 nm in a 10xc3x9710 xcexcm2 area at its Ga-side.
In ranges of progressively increasing preference, the RMS surface roughness of such wafer at its Ga-side is within the following ranges: (1) less than 0.7 nm in a 10xc3x9710 xcexcm2 area; (2) less than 0.5 nm in a 10xc3x9710 xcexcm2 area; (3) less than 0.4 nm in a 2xc3x972 xcexcm2 area; (4) less than 0.2 nm in a 2xc3x972 xcexcm2 area; and (5) less than 0.15 nm in a 2xc3x972 xcexcm2 area.
AIxGayInzN wafers according to the present invention preferably are characterized by a regular step structure at the Ga-side thereof when observed by atomic force microscope.
AlxGayInzN wafers according to the present invention preferably are characterized by that the crystal defects of the AlxGayInzN wafer at its Ga-side constitute small pits with diameters of less than 1 xcexcm. Small pits of such size are readily visible by both atomic force microscope (AFM) and scanning electron microscope (SEM) techniques, while at the same time these pits do not constitute significant damage of the AlxGayInzN wafer surface and therefore do not impair quality of AlxGayInzN crystals subsequently grown thereon.
Such high quality AlxGayInzN crystal wafers are readily manufactured by chemically mechanically polishing (CMP) AlxGayInzN wafer blanks at the Ga-side thereof, using silica or alumina-containing CMP slurry compositions. The corresponding CMP process enables the crystal defects of the AlxGayInzN wafer (evidenced by small pits of less than 1 xcexcm in diameter) to be readily visualized.
Another aspect of the present invention relates to an epitaxial AlxGayInzN crystal structure, comprising an epitaxial Alxxe2x80x2Gayxe2x80x2Inzxe2x80x2N (wherein 0 less than yxe2x80x2xe2x89xa61 and xxe2x80x2+yxe2x80x2+zxe2x80x2=1) film grown on the above-described AlxGayInzN wafer of the invention. Such epitaxial AlxGayInzN crystal structure preferably comprises a wurtzite crystalline thin film, but may be in any other suitable form or structure suitable for specific semiconductor, electronic, or optoelectronic applications. The composition of the epitaxial film may be or may not be the same as the composition of the wafer substrate. The epitaxial AlxGayInzN crystal structure may comprise several epitaxial Alxxe2x80x2Gayxe2x80x2Inzxe2x80x2N films with different composition or doping sequentially grown on the above-described AlxGayInzN wafer of the invention. The epitaxial film may have graded composition, i.e., the composition of the epitaxial film varies with the distance from the interface between the substrate and epitaxial film. As used herein,. the term xe2x80x9cthin filmxe2x80x9d means a material layer having a thickness of less than about 100 xcexcm.
Yet another aspect of the present invention relates to an optoelectronic device that comprises at least one such epitaxial AlxGayInzN crystal structure grown on the above-described AlxGayInzN wafer of the invention.
A further aspect of the present invention relates to a microelectronic device that comprises at least one such epitaxial AlxGayInzN crystal structure grown on the above-described AlxGayInzN wafer of the invention.
A further aspect of the present invention relates to an AlxGayInzN boule that comprises epitaxial AlxGayInzN crystal structure grown on the above-described AlxGayInzN wafer of the invention. A boule is defined as that it can be sliced into at least two wafers. An AlxGayInzN boule can be grown with any suitable method such as hydride vapor phase epitaxy (HVPE), the metallorganic chloride (MOC) method, metallorganic chemical vapor deposition (MOCVD), sublimation, liquid phase growth, etc.
The invention in a further aspect contemplates a method of chemically mechanically polishing (CMP) an AlxGayInzN wafer at its Ga-side, using a CMP slurry comprising:
Abrasive amorphous silica particles having particle size of less than 200 nm;
at least one acid; and
optionally, at least one oxidation agent;
wherein the pH value of the CMP slurry is in a range of from about 0.5 to about 4.
The abrasive amorphous silica particles in the CMP slurry may for example comprise fumed silica or colloidal silica. The amorphous silica particles in the CMP slurry preferably have an average particle size in the range from about 10 nm to about 100 nm. The CMP slurry of the invention in a preferred compositional aspect comprises at least one oxidation agent, e.g., hydrogen peroxide, dichloroisocyanuric acid, or the like.
The pH value of such CMP slurry preferably is in a range of from about 0.6 to about 3, and more preferably is in a range of from about 0.8 to about 2.5.
A further aspect of the present invention relates to a method of chemically mechanically polishing (CMP) an AlxGayInzN wafer at its Ga-side, using a CMP slurry comprising:
abrasive colloidal alumina particles having particle size of less than 200 nm;
at least one acid; and
optionally, at least one oxidation agent;
wherein the pH value of the CMP slurry is in a range of from about 3 to about 5.
The abrasive colloidal alumina particles in the CMP slurry preferably have particle sizes in a range from about 10 nm to about 100 nm.
The CMP slurry of the invention in a preferred compositional aspect comprises at least one oxidation agent, e.g., hydrogen peroxide, dichloroisocyanuric acid, or the like.
The pH value of such CMP slurry preferably is in a range of from about 3 to about 4.
A further aspect of the present invention relates to chemical mechanical polishing (CMP) of the AlxGayInzN wafer at its Ga-side, using a CMP slurry that comprises:
amorphous silica particles having particle sizes of less than 200 nm;
at least one base; and
optionally, at least one oxidation agent,
wherein the pH value of the CMP slurry is in a range from about 8 to about 13.5.
The amorphous silica particles in such CMP slurry preferably comprise fumed silica particles having particle sizes in the range from about 10 nm to about 100 nm, or colloidal silica particles having particle sizes in the range from about 10 nm to about 100 nm.
Bases useful for the practice of the present invention include, but are not limited to, ammonia, alkanolamines, and hydroxides, e.g., KOH or NaOH. Ammonia and alkanolamines are particularly preferred, since they also function to stabilize the CMP slurry.
Such CMP slurry comprises at least one oxidation agent, e.g., hydrogen peroxide, dichloroisocyanuric acid or the like.
The pH value of such CMP slurry preferably is in a range of from about 9 to about 13, and more preferably the pH is in a range of from about 10 to about 11.
A further aspect of the present invention relates to a method of highlighting crystal defects of an AlxGayInzN wafer at it Ga-side to facilitate determination of crystal defect density of such wafer, comprising the steps of:
providing an AlxGayInzN wafer;
chemically mechanically polishing the wafer at its Ga-side, according to one of the above-described CMP methods of the invention;
cleaning and drying the polished AlxGayInzN wafer; and
scanning the wafer with an atomic force microscope or a scanning electron microscope to determine defect density in the wafer.
The CMP process is preferably conducted using an acidic silica slurry as described hereinabove.
Yet another aspect of the present invention relates to a method of fabricating high quality AlxGayInzN wafers, comprising the steps of:
providing an AlxGayInzN wafer blank having thickness in a range of from about 100 xcexcm to about 1000 xcexcm;
optionally reducing internal stresses of the AlxGayInzN wafer;
optionally lapping the AlxGayInzN wafer blank at its N-side, using a lapping slurry comprising abrasives having an average particle size in a range of from about 5 xcexcm to about 15 xcexcm;
optionally mechanically polishing the AlxGayInzN wafer blank at its N-side, using a mechanical polishing slurry comprising abrasives having average particle size in a range of from about 0.1 xcexcm to about 6 xcexcm;
optionally lapping the AlxGayInzN wafer blank at its Ga-side, using a lapping slurry comprising abrasives having an average particle size in a range of from about 5 xcexcm to about 15 xcexcm;
mechanically polishing the AlxGayInzN wafer blank at its Ga-side, using a mechanical polishing slurry comprising abrasives having average particle size in a range of from about 0.1 xcexcm to about 6 xcexcm;
chemically mechanically polishing the AlxGayInzN wafer at its Ga-side, using a CMP slurry comprising at least one chemical reactant and abrasive colloidal particles having average particle size of less than 200 nm; and
optionally mild etching to further reduce internal stresses of the AlxGayInzN wafer and. improve the surface quality,
wherein the resultant AlxGayInzN wafer has a root mean square (RMS) surface roughness of less than 1 nm in a 10xc3x9710 xcexcm2 area at its Ga-side.
The AlxGayInzN wafer blank may be produced by any suitable method, as for example: (1) growing an AlxGayInzN boule and then slicing it into wafer blanks; or (2) growing a thick AlxGayInzN film on a foreign substrate and then separating such thick film from the substrate. The wafer blank may be oriented so that the c-axis is perpendicular to the wafer surface or it may be intentionally slightly misoriented (c-axis not perpendicular to the wafer surface) to facilitate subsequent epitaxy growth, device processing or device design.
The AlxGayInzN wafer blank may be subjected to processing for reducing the internal stress caused, for example, by the disparity of thermal coefficients and lattice constants between such AlxGayInzN wafer and the foreign substrate on which it is grown. Reduction of internal stress may be conducted either by thermally annealing the AlxGayInzN wafer or chemically etching the wafer.
Thermal annealing preferably is carried out at an elevated temperature, e.g., from about 700xc2x0 C. to about 1000xc2x0 C., in nitrogen or ammonia environment for a time of from about 1 minute to about 1 hour.
Chemical etching of the AlxGayInzN wafer functions to remove a layer of surface material from said wafer, thereby relaxing the internal stress of said wafer. It is preferred that the chemical etching process effect a removal of surface material of less than 100 xcexcm in thickness of said wafer, and more preferably less than 10 xcexcm thickness.
The AlxGayInzN wafer can be chemically etched either by a very strong acid at elevated temperatures, e.g., sulfuric acid, phosphoric acid, or combinations thereof, or by a very strong base at elevated temperatures, e.g., molten KOH or NaOH.
Lapping slurry compositions advantageously used in the practice of the present invention may comprise any suitable abrasives, including, but not limited to, diamond powders, silicon carbide powders, boron carbide powders, and alumina powders. Preferably, the lapping slurry comprises diamond powder having average particle size in the range from about 6 xcexcm to about 10 xcexcm. More preferably, two or more lapping slurries lap the AlxGayInzN wafer blank, with each subsequent lapping slurry comprising abrasives of a progressively smaller average size. For example, the AlxGayInzN wafer blank may be lapped by a first slurry comprising abrasives of an average size from about 8 xcexcm to about 10 xcexcm, and then by a second slurry comprising abrasives of an average size from about 5 xcexcm to about 7 xcexcm.
Similarly, the mechanical polishing slurry useful in the present invention may comprise any suitable abrasives, including but not limited to diamond powders, silicon carbide powders, boron carbide powders, and alumina powders. Diamond powders with average particle size in the range from about 0.1 xcexcm to about 3 xcexcm are particularly preferred. The mechanical polishing step may also employ two or more mechanical polishing slurries, with each subsequent mechanical polishing slurry comprising abrasives of a progressively smaller particle size. For example, a first mechanical polishing slurry comprising abrasives of an average size from about 2.5 xcexcm to about 3.5 xcexcm can be used, followed by a second mechanical polishing slurry comprising abrasives of an average size from about 0.75 xcexcm to about 1.25 xcexcm, followed by a third mechanical polishing slurry comprising abrasives of an average size from about 0.35 xcexcm to about 0.65 xcexcm, followed by a fourth mechanical polishing slurry comprising abrasives of an average size from about 0.2 xcexcm to about 0.3 xcexcm, and finally by a fifth mechanical polishing slurry comprising abrasives of an average size from about 0.1 xcexcm to about 0.2 xcexcm.
The CMP slurry comprises at least one chemical reactant, which can be either an acid or a base. When it is an acid, it is preferable to adjust the pH value of the CMP slurry to a value in a range of about 0.5 to about 4; if it is instead a base, it is preferable to adjust the pH value of such slurry to a value in a range of from about 8 to about 13.5.
After the CMP, the AlxGayInzN wafer may be subjected to additional processing for further reducing the stress of the wafer and improving the surface quality. A mild etching is preferred for this purpose. The mild etching may remove some residual surface damage on the Ga-side surface from final CMP polishing while not etch the undamaged surface of Ga-side, thus improve the surface quality. The mild etching can also remove the damage on the N-side surface, thus reduce the stress on the wafer caused by surface damage. This mild etching can also produce a mat finish on the N-side surface. For example, the wafer can be slightly etched in an aqueous solution of base (for example, KOH or NaOH) or an aqueous solution of acid (for example, HF, H2SO4, or H3PO4) at a temperature below the boiling point of the aqueous solution, typically about 100xc2x0 C.
Other aspects, features, and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.